These two brothers are Castor and Pollux's cousins. When the Dioscuri kidnap the lovely Phoebe and Hilaeira, who are engaged to Lynceus and Idas, a rivalry begins that will relieve all four men of their mortal lives. When you think about it, Lynceus and Idas have pretty good reason to be ticked off at their cousins. How would you feel if your cousin rode up, stole your fiancé, and forced him or her into marriage? Yeah, it would suck, right? Some tellings of the tale even explicitly say that Castor and Pollux rape Phoebe and Hilaeiria, which would most certainly give Lynceus and Idas good reason to hate their cousins.
Even in the conflict over the cattle that leads to the death of both pairs of brothers, the Dioscuri are far from innocent. For starters, they all stole the cattle to begin with. After that, though, Idas wins all the cattle for him and his brother by betting that they can finish their steak dinners faster than the Dioscuri. Sure, he's being a little sneaky, because the Dioscuri aren't totally aware that he's a particularly fast eater. But still, a bet's a bet, right? It seems like Lynceus and Idas have some right to be furious with Castor and Pollux they try to steal the cattle back later on.
What's interesting is that even though Lynceus and Idas might seem to be on the right side of this rivalry, they're shipped straight to the Underworld after they die. The Dioscuri, however, are allowed by Zeus to share Pollux's immortality and go on being half-immortal forever. Wow, guess it really pays to be related to the king of the Gods, right? In their relationship with Lynceus and Idas, we see that the Dioscuri definitely aren't as perfect as they might seem when you first meet them.